416 THE CONTRACTILE VACUOLE 



that orientation of surface molecules occurs at the interface between two 

 different phases (provided, of course, that at least one phase contains 

 polar molecules), whether the system is composed of oil and water or 

 protoplasm and water. The converse of this is equally true; since the 

 very existence of this oriented layer depends on the presence of tivo 

 different phases, the removal of one phase necessarily results in disintegra- 

 tion of the membrane. Whether or not this surface layer of oriented 

 molecules actually comprises the true physiological membrane may be 

 subject to debate, but the importance of such a membrane is obvious, 

 since it not only separates the organism from its surroundings but at 

 the same time provides the only means of communication between the 

 interior and the exterior of the cell. 



If one accepts the idea of a physiological membrane as a layer of 

 oriented molecules at a phase boundary, as outlined briefly above, then 

 it necessarily follows that any cell vacuole which contains a fluid differ- 

 ent from cytoplasm must be surrounded by such a membrane. In the 

 light of the information available at the present time, membranes around 

 protozoan contractile vacuoles probably should be considered as tem- 

 porary; although it is not inconceivable that in some forms the new vac- 

 uole may be formed with such rapidity, in the midst of oriented mole- 

 cules remaining after systole, that dispersion of the membrane does not 

 have time to occur before the second phase is present again. If such condi- 

 tion obtains, then the membrane may be considered as having a greater 

 or less degree of permanence. There are numerous references to such 

 physiological membranes in the literature on contractile vacuoles, so it 

 appears that the idea has gained rather wide acceptance. The controversy is 

 not so much concerned with such membranes as with the presence or 

 absence of morphological, and hence permanent, membranes. 



Before leaving the subject of physiological membranes, there are sev- 

 eral phenomena which may be discussed profitably with this concept in 

 mind. Repeatedly authors speak of the coalescence of accessory vacuoles 

 to form contractile vacuoles. Taylor (1923) refers to this, and considers 

 coalescence to be due to a reversion of the gel state of the surrounding 

 film to the sol state. He further states that vacuoles are surrounded by 

 highly viscous boundaries of endoplasm, and that the consistency of the 

 papilla pulsatoria strikingly resembles that of the endoplasmic boundaries. 

 Without raising the question as to whether or not coalescence of accessory 



